Small implantable ion-sensitive electrodes which may be used to continuously measure electrolyte balance in body fluids would be very valuable to the clinician as well as to the biophysicist. The proposed solid-state hybrid ion-sensitive device is being developed for several patient monitoring applications including measurement of gastric pH and intravascular calcium-ion concentration, as well as for use in the study of cationic exchange in muscle. The proposed electrodes will be fabricated by a combination of thick-film and thin-film technologies, presently used in the silicon integrated circuit industry. The accuracy and stability of the new devices will be tested in vitro using standard chemical procedures, and comparisons will be made with commercially-available ion-sensitive electrodes. The fabrication procedures to be employed will permit low cost volume production of the devices with extremely high uniformity. Thus, the electrodes will fall into the category of disposable devices.